The diagnosis of invasive fungal infections (IFI) is a major unmet medical need. As many as 15% of patients with allogeneic hematopoietic stem cell transplant develop IFI, mostly caused by Aspergillus. Patients with prolonged neutropenia (due to any cause) or immunosuppression (due to transplants or treatment with corticosteroids) are at particularly high risk.
Currently, the diagnosis of IFI relies on a combination of clinical and laboratory criteria. The criteria were developed as an international consensus and the certainty of the diagnosis ranges from definite (detection of the fungus in tissue) to probable or possible. Because definite diagnosis requires biopsy and visualization of the organism in tissue, the majority of patients with IFI fall into the probable or possible categories. Even when a histologic diagnosis is made, the mold cannot be definitively identified because molds grow as hyphae in tissue and do not form spores. Since the anti-fungal susceptibility of different genera and species differs, specific diagnosis is of great clinical importance. There are now effective and relatively non-toxic therapies available for IFI, especially those caused by Aspergillus fumigatus. Thus, the diagnostic limitations have profound effects on the treatment of IFI.
It is challenging to diagnose IFI non-invasively, as it is difficult to successfully culture fungi from blood or respiratory secretions. Complicating the problem, fungi are common in the environment and, thus, a positive culture could be due to environmental contamination. Two non-invasive assays are available. An assay for circulating galactomannan, a constituent of the Aspergillus cell wall, has recently been approved by the FDA. As fungi other than Aspergillus do not have galactomannan in their cell walls, a negative test does not exclude IFI. Furthermore, the sensitivity and specificity of the galactomannan assay vary greatly from study to study for reasons that include technical differences in how the test is performed in the U.S. and Europe, the incidence of aspergillosis in the population tested, and low sample size. A test is also available for circulating glucan, a component of all fungal cell walls. This test should be of wider utility than that for galactomannan, although it has not been as widely studied as the galactomannan assay.
Invasive candidiasis can occur in severely immunosuppressed individuals and in patients who have central venous catheters, especially if they are on systemic antibiotics and/or parenteral nutrition. The diagnosis of invasive candidiasis currently rests primarily on detection of the organism in blood cultures. In the correct clinical setting, positive cultures from respiratory secretions and urine raise the suspicion of systemic infection. The diagnosis of the species of Candida is important because Candida albicans is much more susceptible to fluconazole than other species of Candida, especially C. krusei. Species identification of the organism can take up to 10 or more days, although the presence of Candida can be determined fairly quickly (1-2 days).
There have been many attempts to develop a diagnostic test for fungal DNA. Blood and bronchoalveolar lavage fluid have been the main fluids studied. Although different DNA extraction methods, various target genes and primers, and a variety detection methods and analytical techniques have been used, none of the published techniques have shown a strong enough correlation with clinical diagnosis to establish any as a preferred approach.